Stack Exchange Network

Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [luminosity]

Luminosity is the energy radiated from a body per second.

-2
votes
0answers
26 views

No dark matter, weight of the universe measured by luminosity is unaccounted/distorted relections?

If calculating the universe's mass by luminosity, how do we account for all the combined reflections from our planets or planets from other solar systems, and throughout all of time and space? And can ...
0
votes
1answer
18 views

Luminostiy, Flux, Intensity definitions and meaning

I am taking astrophysics course and we are constantly talking about Luminosity,Brightness,Intensity,Flux etc. I thought that I understand the idea properly but I see now that I dont. Intensity is the ...
3
votes
1answer
80 views

Why is the relation between luminosity distance $d_L$ and comoving distance $\chi$ $d_L=\chi/a$?

The textbook (Scott Dodelson, Modern Cosmology, Section 2.2 Distance, Page 35-36) states as follows: Another way of inferring distances in astronomy is to measure the flux from an object of known ...
0
votes
1answer
39 views

Luminosity of a white dwarf

I have a very simple question, yet i am confused.Can someone tell me why in an H-R diagram the white dwarfs are at the bottom left? As in I understand that they would have a high temperature when they ...
1
vote
1answer
116 views

What is the closest distance for an undiscovered old neutron star and how long would it take to get here? To help people scared of neutron stars

This is to help people who are scared of neutron stars. My questions are What is the faintest a neutron star could be How close could it be and not yet be detected How long would it take to get here ...
0
votes
0answers
26 views

Stephan Boltzmann law

I was preparing for INAO when I came across the application of Stephan Boltzmann law...it's says that energy absorbed per unit time by a planet is Flux times πr^2. But why did we multiply with πr^2 or ...
0
votes
1answer
29 views

Do bigger stars emit higher amplitude waves?

So I’ve been reading about luminosity and I get the concept of the size and temperature determining the intrinsic brightness and total energy emission of the star. I also understand how the further ...
1
vote
0answers
48 views

Where can I find values for apparent brightness of stars?

I'm in high school and doing a project. I want to calculate the distance to stars using their luminosity and apparent brightness, from the equation $b=\frac{L}{4 \pi d^2}$. I have found values for ...
1
vote
0answers
96 views

How to calculate luminosity in g-band from absolute AB magnitude and luminosity distance?

How can I calculate the (non-bolometric) luminosity $L$ of a galaxy (or a star for that matter) over a given band from its AB apparent magnitude $m_{AB}$ over that band and its luminosity distance $...
0
votes
1answer
168 views

Mass Luminosity Relation Review

I have read that exists a relation between the luminosity and the mass of a star $$\frac{L}{L_\odot}=\left(\frac{M}{M_\odot}\right)^a$$ with $a$ depending on the mass $M$. I have looked for a ...
1
vote
1answer
663 views

Are lumens linearly additive?

If I have two light sources side by side emitting $1000~\rm lumens$ each, am I going to have the same amount of light emitted in an enclosed space (e.g. a room) as if I had a single light source that'...
0
votes
1answer
79 views

Illuminance at a given distance from a diffuse emitting light box

Therapeutic light boxes purport to deliver 10,000 lux to a user, typically sitting ~24" from a box. I was wondering how many lumens are necessary to achieve this illuminance, e.g. what lamps would be ...
0
votes
2answers
36 views

Safe Laser displays

I hope this is in the right stack exchange. Here's my question, assuming that all blue lasers are too risky to the eye to risk [using], is it possible to use only low-energy red and green lasers and: ...
3
votes
1answer
363 views

Maximum light intensity?

From what I had learned and observed, every image (light per certain area,let it be cm^2 here) can be projected onto a small or large area without any natural obstruction from intensity. So, if light ...
1
vote
2answers
1k views

How the Hertzsprung-Russell Diagram allows us to calculate distance to stars

I understand how to interpret a H-R diagram, in the sense that I know that the upper right top corner is occupied by cool stars, but they are very luminous so they must be big; and the bottom left ...
-1
votes
2answers
245 views

How to calculate energy per range of nm

$K=hc/\mathrm{wavelength}$ is the usual for finding energy for one particular wavelength. But how would one calculate a range of wavelengths incident on a particular sized surface area? Because no ...
1
vote
0answers
37 views

Object that is visible only at certain light intensity

I have a lux meter, by which we can measure light intensity. We have one box, in which we inject the light from a hole. What we want is, if light intensity for e.g. Lux value is between 400 to 450 ...
0
votes
1answer
604 views

Converting lux to watts with known wavelength

I was wondering if there was a possible way to convert lux to watts accurately if the wavelength is known. I know that for the sun there is an approximate conversion of 0.0079W/m$^2$ per Lux using ...
4
votes
2answers
230 views

How would a nuclear explosion look in vacuum?

Approximately, how does the specific luminosity, $L_\nu(t)$, of a nuclear explosion in a vacuum look as a function of frequency and more importantly, time? Assume the observer is very far away. If ...
0
votes
1answer
130 views

Schechter' luminosity function

I need your help. I need show that the Schechter' Luminosity Function with: $\alpha = -1.5$ the limited-magnitude of a survey will be detected from a sample of galaxies with this characteristics: ...
2
votes
2answers
154 views

Relation between baryonic mass and luminosity [duplicate]

I have one simple question: How do we know that dark matter is not just ordinary matter which is simply not luminous? For example if we would take a black rock and put it somewhere in the universe, ...
0
votes
2answers
85 views

Stars with same colour

This is the problem I'm trying to solve. When it says they have the same colour, does that mean that they have the same temperature?
0
votes
2answers
879 views

Relationship between Stefan-Boltzmann Law and Radiance

In a textbook, the equation for radiance is given I am struggling to understand the relationship between the two equations. From what I know, multiplying the value obtained by the Stefan-Boltzmann ...
14
votes
5answers
1k views

What is “white light” ? Uniform wavelengths or uniform frequencies ?

Suppose you have a mixture of electromagnetic waves of wavelengths spreaded on the visible spectrum only (from $\lambda_{\text{min}} \sim 400 \, \text{nm}$ to $\lambda_{\text{max}} \sim 700 \, \text{...
0
votes
2answers
104 views

How to define the light “color” from a given spectral distribution?

The following question may be naive and incomplete in some way I don't know. I'm not a specialist on spectroscopy, colours and light curves, color spaces, etc. Suppose you have a simple power-law ...
0
votes
1answer
543 views

Radiation pressure in terms of luminosity?

My class slides have given me the following equation for $P_{rad}$: $P_{rad}=\frac{4\sigma T^4}{3c}$, where $\sigma$ is the Stefan-Boltzmann Constant. I know that radiation pressure is pressure on a ...
4
votes
0answers
173 views

Distribution of wavelengths of monochromatic light sources in expanding FLRW universe

I'm looking for the distribution of all wavelengths (or frequencies) of light that a stationary observer would receive at his location (at $r = 0$ and time $t_0$), from light sources emitting a single ...
2
votes
1answer
108 views

Can the luminosity distance be greater than the particle horizon distance?

While writing my notes on cosmology, I found this little puzzle, which isn't described in any of my books on general relativity. Consider a flat space ($k = 0$) filled with dust-like matter, of ...
17
votes
1answer
483 views

Olbers' paradox in a closed space

I'm looking for a proper interpretation of the following, which is a variant of the Olbers' paradox. Consider a spatially closed empty universe ($k = 1$) with a positive cosmological constant $\...
1
vote
0answers
37 views

what is the telescopic retinal illuminance of a star?

What is the retinal illuminance (in trolands) or luminance (in $cd/m^2$) of a star image viewed through a telescope? I'm looking for a formula that will calculate this value for input values of star ...
1
vote
0answers
53 views

Number of stars with apparent flux

The stars in our Galaxy have luminosities ranging from $L_{\text{min}}$ to $L_{\text{max}}$. Suppose that the number of stars per unit volume with luminosities in the range of $L$, $L+dL$ is $n(L)dL$. ...
1
vote
2answers
325 views

Redshift Versus Luminosity

I understand that there is a relation between the proper distance of a cosmic object and its "measurable" redshift, i.e. once you know the value of the redshift parameter z, then you actually know how ...
0
votes
0answers
55 views

Interpreting LED output distribution specification

I have specifications for an LED which gives the following graph of spacial output distribution. I am not sure how to interpret this. As I see it, it could be one of two ways. To make explanations ...
0
votes
1answer
116 views

What will happen in this LASER setup?

If I have 3 rectangular mirrors, say of dimensions length of 1 meter and width of 0.5 meter and I arrange them in the form of an equilateral triangle, which is of side 0.5 meter.(The mirrors are ...
1
vote
0answers
41 views

Where does this Gamma formula come from?

This is the derivation of formula of Gamma Correction for CRT monitors. I am not finding this in any book or web-site. Books seem to derive this formula in different ways. Can anyone please tell me ...
2
votes
1answer
653 views

Convert from Relative Magnitude to Mass

I have data which gives me the magnitude density (${\rm mag}\,{\rm arcsec}^{-2}$) of M31 as a function of radius. How can I convert these data to the (enclosed) mass at a given radius (for velocity ...
1
vote
0answers
46 views

Lorentz factor correction for luminosity

I am looking at the physics behind gamma ray bursts, working through a comprehensive review Kumar & Zhang (2014) (arXiv link). On page 92, Eq (111) the luminosity or a relativistically expanding ...
0
votes
1answer
235 views

Distance and luminosity distance

In my cosmology lecture notes I read that a way to measure distances in cosmology is to use standard candles and the comparison between "absolute luminosity" of the candle and the apparent luminosity. ...
2
votes
2answers
1k views

Explain relationship between angular diameter distance and luminosity distance, Etherington Theorem

I have a question relating to the Etherington Theorem. The luminosity distance is defined by the equation for flux, i.e. $F=\frac{L}{4\pi D_L^2}$ where flux is in units energy per unit time (...
1
vote
1answer
95 views

Why do we ignore the Flux - Luminosity relation when deriving the Eddington Luminosity?

I was just doing the derivation for the Eddington Luminosity by considering the momentum equation, $$ -\frac{Gm}{4\pi r^4} = \frac{dP}{dm} $$ and the radiative transport pressure $$ \frac{dT}{dr} = ...
11
votes
1answer
194 views

Eclipse of the sun - Intriguing luminosity curve

I've just recorded the luminosity during the sun eclipse, here is the resulting curve : Only green curve is relevant, blue curve should be ignored (it's actually the temperature). The sky was ...
0
votes
1answer
300 views

Can incandescent light bulb can be brighter if fixed power?

I found that luminous efficacy of 60 W tungsten incandescent bulb is 14.5 lm/W. Is this value defined by design details listed above or it depends on something else too? Can we design 60 W tungsten ...
0
votes
1answer
7k views

Relation between number of photons and energy?

Please can anyone explain it. If number of photons are increased will it increase the providing energy. Suppose, we are sending a limited number of photons each carrying energy. We have a energy ...
2
votes
0answers
38 views

Why don't electrons return to their ground state immediately after photoexcitation? [duplicate]

In terms of photoluminescence, why don't the electrons, which have been excited by photons earlier, immediately fall down to their ground state and reemit a photon? In other words, why does 'glow-in-...
7
votes
2answers
4k views

Position of Neutron Stars in H R diagrams

Why is that neutron stars are never depicted in a Hertzsprung-Russell diagram? They can be placed in the bottom left corner but you will never find any diagram in literature showing neutron stars.
1
vote
1answer
104 views

Is there uncertainity of position of the perfectly homogenous radiating body?

I heard the standard interpretation of Heisenberg Uncertainty Principle: Just the measurement affects the position of the body because always you want to see a body (=to measure the position), you ...
4
votes
2answers
762 views

Luminance from multiple light sources

I have a LED diode: Given Luminance intensity [I] of $1.85$ cd. I want to have Luminance [B] from the light source of $75 \ {\rm cd/m^2}$. From $B=I/S$, where S is light source plane area, I get $S=...
1
vote
1answer
67 views

Regarding luminosity monitor of particle collider

I am sorry if my question is completely wrong or foolish I would feel glad to be corrected On the wiki page for Bhabha Scattering, it is mentioned that it is used as luminosity monitor in many ...
0
votes
1answer
219 views

How does intensity differ from apparent luminosity in the context of photometry?

Context: photometry in astronomy. Background: The total luminosity $L$ of a star is the energy that radiates in all spatial directions in all wavelengths and is given by the following formula, where $...
6
votes
1answer
124 views

Question on luminosity of a radiant source and its dependence on temperature, and involvement of Doppler Effect

A few days ago, I happened to go through the chapters on Radiation, and Photometry, studying them at quite an elementary level. I studied Wien's displacement law, and the dependence of luminous flux ...